In some areas of semiconductor manufacturing, ultra highly-integrated devices may be implemented having a minimized design rule, which may require a circuit critical dimension (CD) to be minimized. As structures of semiconductor layers and patterns may be relatively complicated, connecting (e.g. contacting) layers to each other may be necessary. Connecting layers to each other may use contacts of a metallic material (e.g. tungsten (W) or copper (Cu)).
Some semiconductor device sizes have become relatively small due to nanotechnology, which may require relatively fine contacts. There are different kinds of fine contacts, which may be classified into direct contacts, word line contacts, bit line contacts, plate electrode contacts, reflow contacts, and other similar contacts. Direct contacts expose a surface of a semiconductor substrate. Word line contacts expose the top of a gate electrode.
FIGS. 1A through 1F illustrate a method of forming a reflow contact. As illustrated in example FIG. 1A, a prime process may be performed by putting a wafer substrate 1 on a hot plate of approximately 80° C. and treating it with hexa-methyl-di-silazane (HMDS) for approximately 10 seconds. In a prime process, the adhesive strength between a photo resist (PR) pattern and the wafer substrate 1 may be increased, to prevent the PR pattern from shifting (e.g. do to lack of traction) during manufacturing processes.
As illustrated in example FIG. 1B, a positive PR 2 may be formed on the top surface of wafer substrate 1. Positive PR 2 may have a thickness between approximately 0.2 μm and 0.5 μm. A solvent in PR 2 may be removed by performing a soft bake on the hot plate between approximately 90° C. and 110° C. for approximately 90 seconds.
As illustrated in example FIG. 1C, PR 2 may be exposed with energy (e.g. between approximately 5 mJ/cm2 to 50 mJ/cm2) through mask 3. A region of PR 2 that is not exposed with energy may be covered by a pattern of Cr 4 on mask 3.
As illustrated in example FIG. 1D, PR 2 may be subjected to a post-exposure bake (PEB) on a plate (e.g. having a temperature between approximately 90° C. and 130° C. for approximately 90 seconds). As illustrated in example FIG. 1E, the PR 2 may be developed by an alkali solution, such that only an unexposed region of the PR 2 remains, thus resulting in a PR pattern.
As illustrated in example FIG. 1F, PR pattern 2 may be flowed by a flow bake. A flow bake may have a temperature higher than a soft bake. By the flow bake, the size of a contact hole 5 in PR 2 may be reduced. The manner of the flow bake may be different depending on the type PR 2 used. As an example, a flow bake may be performed at temperatures between approximately 120° C. and 150° C. for approximately 90 seconds.
A process illustrated in example FIGS. 1A through 1F may limit the size of a contact hole to more than 90 nm. Further, scum may remain when patterning a contact hole using the process illustrated in example FIGS. 1A through 1F, which may cause non-uniformly sized contact holes that may not be reproducible. For example, if the intensity of light in an exposing process does not reach a certain threshold, a PR may not be precisely formed, causing formation of contact holes having undesired sizes.